In general, a first wall (a wall directly facing on plasma) of nuclear fusion apparatus interacts with heat radiation from plasma and plasma constituent particle leaked out from plasma confinement region, and as the result the first wall is eroded to release first wall constituent materials as an impurity in the plasma. Therefore, the first wall is regarded as a main source of plasma impurity. On the other hand, in order to elevate a plasma temperature as higher as possible in a nuclear fusion reaction, it is necessary to lower an energy loss by impurity from plasma to the utmost. It is known that the energy loss by impurity increases with atomic number of impurity element under the same content. Accordingly, it is now planned to use a material of small atomic number such as carbide, boride, etc. as a first wall constituent material of nuclear fusion apparatus, and among them, titanium carbide having a stoichiometric composition, i.e. Ti/C=1 is the most hopeful material proposed. However, from a view of mechanical processability and mechanical strength., etc., it is difficult to constitute a first wall with titanium carbide only.
Thereby, a material excellent in thermal and mechanical properties such as molybdenum material is used as a base and a titanium carbide film of several tens .mu.m in thickness is deposited onto the surface of base, and is used as a titanium carbide deposited first wall. However, since the first wall is eroded during the operation of nuclear fusion apparatus due to the cause as described above, eroded portions have to be repaired with a fresh titanium carbide film on all occasions. In this case, if the first wall of eroded portion is removed from the apparatus and repaired on all occasions, the operation and maintenance of nuclear fusion apparatus is very troublesome and the operation cost becomes high. Therefore, it is very desirable in operation and maintenance of nuclear fusion apparatus using a titanium carbide deposited first wall to develop such a method (i.e. in situ coating method) that the eroded portion can be repaired with a fresh titanium carbide film in vacuo without removing the first wall out of the apparatus. Now then, in a nuclear fusion apparatus which is now under contemplation, when a titanium carbide film is deposited onto the surface of first wall, a distance that a deposition particle (metallic titanium particle) flying out from a titanium vaporizing source has to fly to reach the surface of first wall is about 1 m, and this value increases more and more with a size of nuclear fusion apparatus to grow larger.
In general, the flying distance of deposition particle is decided by a rate of collision and scattering of deposition particle and atmospheric gas particle in space.
The average distance of particle flying from a collision to the next collision (mean free path) can be found by the kinetic theory of gasses. According to the theory, it is necessary to make an atmospheric gas pressure below 5.times.10.sup.-5 Torr in order that the mean free path is above 1 m. Therefore, if it is intended to fly a deposition particle above 1 m for the purpose of depositing a titanium carbide film onto the surface of first wall of the above mentioned nuclear fusion apparatus, it is necessary to maintain an atmospheric gas pressure below 5.times.10.sup.-5 Torr during deposition.
Up to now we have a cathodic sputtering method, a reactive ion plating method, a gas phase reaction method and a reactive vacuum deposition method as a method of depositing out a titanium carbide film on a base. However, in the cathodic sputtering method, the reactive ion plating method and the gas phase reaction method, it is necessary to make the pressure of atmospheric gas such as reaction gas, operation gas, etc. above 10.sup.-3 Torr in time of performing deposition, and so these methods cannot be used as an in situ coating method for the first wall of nuclear fusion apparatus for the above mentioned reason.
On the other hand, the reactive vacuum deposition method which has hitherto been known is a method of using ethylene as a reaction gas but, in practice, is very difficult to be put to practical use since the depositing velocity of titanium carbide film onto a base becomes very low when lowering the ethylene gas pressure below 5.times.10.sup.-5 Torr.